Thin films may be grown on the surface of substrates by several different methods. These methods include vacuum evaporation deposition, Molecular Beam Epitaxy (MBE), different variants of Chemical Vapor Deposition (CVD) including low-pressure and organometallic CVD and plasma-enhanced CVD, and Atomic Layer Epitaxy (ALE), which has been more recently referred to as Atomic Layer Deposition (ALD) for the deposition of a variety of materials.
In ALD, the sequential introduction of reactant species (e.g., a first precursor and a second precursor) to a substrate, which is located within a reaction chamber, is generally employed. Typically, one of the initial steps of ALD is the adsorption of the first reactant on the active sites of the substrate. If one or more elements of the film being deposited are included in the reactant, it can also be referred to as a precursor. Conditions are such that no more than a monolayer forms so that the process is self-terminating or saturative.
For example, the first reactant or precursor can include ligands that remain on the adsorbed species, which prevents further adsorption. Accordingly, deposition temperatures are maintained above the reactant condensation temperatures and below the reactant thermal decomposition temperatures within the so-called ALD window. This initial step of adsorption is typically followed by a first purging stage, where the excess first reactant and possible reaction byproducts are removed from the reaction chamber.
The second reactant is then introduced into the reaction chamber. The first and second reactants typically react with each other. As such, the adsorbed monolayer of the first reactant reacts instantly with the introduced second reactant thereby producing the desired thin film. This reaction terminates once the adsorbed first reactant has been consumed. The excess of second reactant and possible reaction byproducts are then removed by a second purge stage. The cycle can be repeated to grow the film to a desired thickness. Cycles can also be more complex. For example, the cycles can include three or more reactant pulses separated by purge and/or evacuation steps. Sequential pulses of reactants are separated both temporally and spatially to avoid gas phase reactions.
Ideally, in ALD, the reaction chamber design should not play any role in the composition, uniformity or properties of the film grown on the substrate because the reaction is surface specific. However, rather few reactants exhibit such ideal or near ideal behavior. Factors that may hinder this idealized growth mode can include time-dependent adsorption-desorption phenomena, blocking of the primary reaction through by-products of the primary reaction (e.g., as the by-products are moved in the direction of the flow, reduced growth rate downstream and subsequent non-uniformity may result, such as with the corrosive by-products of surface reaction TiCl4+NH3→TiN+ by-products, total consumption (i.e., destruction) of the second reactant in an upstream portion of the reactor chamber (e.g., decomposition of the ozone in the hot zone), and uneven adsorption/desorption of the first precursor caused by uneven flow conditions in the reaction chamber.
Cross-reactions between reactants or breakdown reactions of the reactants before reaching the reaction chamber may also pose problems for obtaining uniform thin films. For example, issues have been identified where TiCl4 is contacted with elemental Ti, yielding TiCl3 or Ti2Cl6 molecules that cause complications during TiO2 or TiN deposition. Additionally, trimethyl aluminum (TMA) has been known to decompose at least partially when delivered into the process chamber via the same pathway as used for some metal chloride reactants.
Such problems have been partially alleviated with the use of a showerhead-type apparatus used to disperse the gases into the reaction space, such as disclosed in U.S. Pat. No. 4,798,165. The showerhead-type apparatus, as found in U.S. Pat. No. 4,798,165, may be positioned above a substrate so that the reactant vapors and purge gases flow through apertures that are located on the showerhead and the gas flow may be directed perpendicular to the substrate. However, in such a configuration, in the course of time the reacted gases may form a film in the apertures and the apertures may become blocked. Such blockage may result in uneven deposition of layers onto the substrate and formation of particles in the showerhead that can contaminate substrates.
A single body injector and deposition chamber has been disclosed in U.S. Pat. No. 6,200,389, where the injector includes a front, back, top, bottom and end surfaces. The injector further includes a first elongated passage formed in the injector and extending between the end surfaces. One of the end surfaces is closed. A chemical delivery line leads to the end of the elongated passage. A distribution channel that extends between the elongated passage and the gas delivery surface is formed in the injector. The gas from the chemical delivery line is intended to flow along the distribution channel out the injector. However, the reaction space sides of channels or passages are especially subject to becoming blocked over time. Additionally, reactant gases cannot reach the entire substrate surface because of a gas flow curtain effect between adjacent injectors. The susceptor or injectors must be moved sideways back and forth during the deposition to reach substrate areas that would otherwise not be exposed to reactant vapors. Need for mechanical movement complicates the deposition method and the construction of the deposition chamber.
Another problem present in the prior art is that, generally, a large distance exists between the reactant in-feed apertures and the exhaust or outlet of the reaction space. For instance, in a typical embodiment wherein a circular outlet is placed around the susceptor plate near the wafer edge and a showerhead plate is placed above the wafer, gases flow from the center of the wafer towards the edge of the wafer. The concentration of reaction byproducts in the gas phase increases towards the edge of the wafer. This becomes increasingly problematic when, for example, hydrogen chloride (HCl) or other corrosive agent is generated as a reaction byproduct. HCl is generated from reactions between metal chlorides and water or between metal chlorides and ammonia and may re-adsorb on the wafer surface and block reactive surface sites. The re-adsorption rate on the surface is a function of the HCl concentration in the gas phase. In such cases the growth rate of the thin film tends to decrease towards the edge of the wafer.
Thus, there is a need for an improved apparatus and method for depositing thin layers that addresses the problems described above.